1. Field of the Invention
The present invention relates to a device as well as a method for detecting scattered light signals.
2. Background Information
Particularly in the field of fire detection devices, smoke detectors which operate on optical principles are known, whereby a scattered light area, in which particles possibly distributed in the air could be present, is subjected to light from a light source. Such particles can be caused for example by dust particles or tobacco smoke particles, but also room fires, the detecting of which is imperative. Outside of the direct optical path of the light emitted from the light source, conventional devices provide for the arranging of optical sensors such as for example photodiodes, photoresistors or the like having an correspondingly associated amplifier circuit. The sensors detect any light there may be scattered by the particles and issue an alarm signal upon for example a specific threshold being exceeded.
Furthermore, systems for classifying different types of particles are known; i.e. in particular systems for classifying different types of fire on the basis of particle properties. For example, a device is known from printed publication EP 2281 286 A1 which enables differentiating between dust particles and those particles which develop during fires. In addition to scattered light sensors (optical sensors), such conventional systems for classifying particle type usually also utilize other types of sensors—for example gas sensors or the like.
The cited conventional devices have the disadvantage of either, in the case of relatively inexpensive configuration, classification according to different kinds of particles only being unreliably possible and with no effective variable disturbance detection and/or suppression, or that relatively expensive sensor technologies need to be used such as for example gas sensors or the like. This drives up the costs and the circuit complexity.
Moreover gas sensors in particular have the disadvantage of needing relatively high energy.
The present invention is based on the objective of further developing a conventional device for detecting scattered light signals such that it can be simply and economically configured and manufactured and the detection accuracy improved. The energy consumption is additionally to be reduced.
This objective is accomplished by a device in accordance with independent claim 1 as well as a method in accordance with independent claim 24.
The dependent claims set forth advantageous further developments of the inventive solution.
The invention is based on the following basic knowledge:
The basic principle behind devices which work optically in detecting scattered light signals, particularly in smoke detectors and the like, is capitalizing on the different scattering characteristics of different types of particles distributed in the ambient air. The ambient air hereby constitutes a carrier fluid in which the particles, usually meaning solid but also definitely including liquid microparticles, are distributed.
Depending on the relationship of particle size to the wavelength of the light to which the scattered light area is exposed, different reflecting and scattering mechanisms take effect with different particles or types of particles. While it can be expected under certain conditions of particle size to incident light wavelength that scattered light will be observed in all spatial directions from a particle, other intensity distributions per reflecting and/or scattering particle result under other conditions of wavelength to particle size, for example solid angle-related or polarization-related intensity distributions.
In other words, the solid angle-related scattered light distribution of a particle onto which a light beam illuminating the particle falls is not only dependent on the wavelength of the incident light, but also as the case may be on the viewing angle, the particle size, the refractive index of the particle medium as well as the polarization of the incident radiation.
In the range of very small particles, in each case relating to the wavelength of the excitation light, an elastic scattering mechanism of the incident electromagnetic waves, known as Rayleigh scattering, generally predominates. In one range within which the wavelength of the energizing light corresponds approximately to the particle size, the scattering mechanism of the elastic scattering of the incident electromagnetic waves can be described by the Mie theory which, while describing an accurate solution to the scattering process, requires presupposition of the particle geometry (spherical particles). With further increasing particle size, the scattering can be described by conventional particle geometrical refraction.
In the realm of Rayleigh scattering and in the realm of Mie scattering, the scattering intensities of the radiation scattered at the particles are functions of, among other things, the solid angle, the particle size (particle radius), the polarization plane, the scattering angle and the complex refractive index of the suspension medium; i.e. in particular air.
The spatial distribution of the light scattered by a particle itself has intensity profiles which are dependent on the viewing direction. During the scattering process, particularly within the realm of Rayleigh scattering and Mie scattering, the interacting components of diffraction, refraction and reflection on the respective scattering particle all play a part in these intensity profiles. Due to this interacting scattering process, not only are the intensity profiles directionally dependent, but the scattering intensities then also vary in their respective polarization directions.
Also playing a role in the detection of scattered light is the fact that, for example with output-based scattered light detection, the aperture of the optical sensor employed is finite. It is hereby then also necessary to take the spatial detection angle into consideration.
The above indicated interaction between different components during the scattering process thus comprises the interaction of diffraction, refraction and reflection at the particle. Hereby, and due to the restrictions of an optical sensor with respect to the spatial detection angle and due to the dependency of, among other things, the particle radius, the wavelength of the incident light, the refractive index of the surrounding medium, the scattering angle and the polarizing angle, the intensity profiles of the scattered light scattered by different types of particles depend particularly on the positioning of the sensor relative to the scattered light area and any polarizing filter there may be in front of the sensor.
The circumstance of the compositions of the particles which develop for example upon a certain type of fire exhibiting a characteristic distribution is hereby capitalized on, whereby superpositioning the different scattering mechanisms or scattering characteristics respectively in the scattered light area likewise yields respective characteristic, position-dependent and polarization-dependent intensity distributions. In other words, the intensity of the scattered light measured at a specific location about the scattered light area in relation to the time over which the particles develop; i.e. during the course of a fire for example, exhibits a location-related and polarization-related characteristic pattern.
While there can still be collisions between characteristic patterns related to different particle types, and thus different types of fire, in the case of just one measuring point about the scattered light area with only one polarization, the probability of such pattern-related collisions drops as the measuring points and/or the detected polarization directions increase.